1. Field of the Invention
This invention relates to an apparatus of receiving and decoding signals in an apparatus for coding and transmitting video and sound signals in a CATV system, pay TV system or the like, and more particularly to the decoding of sourd signals.
2. Description of the Prior Art
In a pay TV system or the like, video and sound signals are transmitted by coding so as not to be accessed by nonsubscribers, and they are decoded by subscribers, so that normal picture and sound may be reproduced. Regarding this video and sound coding means, various methods had been proposed so far, but they have had their own problems.
For example, the method of always inverting black and white colors of the video signal only, the method of compressing the synchronizing signal, and the method of inverting the synchronizing signal were easy to decode, and the signals were easilly accessed illegally. The method of eliminating the synchronizing signal was unstable in the picture because the jitter of the reproduction synchronizing signal was left over. The method of eliminating the color burst signal was inaccurate in the phase of the reproduction color burst signal and unstable in reproducing colors.
Or, of the methods of coding sound signals, the method of scrambling a PCM sound signal was broad in the bandwidth when another sound carrier was used and was unsuited to an FM broadcast such as satellite broadcasting system, and its decoding unit was expensive. In the method of transmitting sound by plurality carriers and changing them over randomly, the decoding unit was complicated, and it was difficult to balance the stereo sound reproduction.